Josephson frequency singularity in the noise of normal-metal-superconductor junctions

A singularity at the Josephson frequency in the noise spectral density of a disordered normal-metal-superconductor junction is predicted for bias voltages below the superconducting gap. The nonstationary Aharonov-Bohm effect, recently introduced for normal metals [G. B. Lesovik and L. S. Levitov, Phys. Rev. Lett. 72, 538 (1994)], is proposed as a tool for detecting this singularity. In the presence of a harmonic external field, the derivative of the noise with respect to the voltage bias reveals jumps when the applied frequency is commensurate with the Josephson frequency associated with this bias. The height of these jumps is nonmonotonic in the amplitude of the periodic field. The superconducting flux quantum enters this dependence. Additional singularities in the frequency dependent noise are predicted above gap. [S0163-1829(99)00138-1].